This invention relates to a process for hydrogenating a carboxylic acid group in a fluorine-containing alkyl, cycloalkyl or benzene carboxylic acid to the primary alcohol group, in the liquid or vapor phase, in the presence of a solid rhodium or iridium catalyst, employed as the metal, metallic oxide, or mixture thereof.
Fluorine-containing alcohols are useful as solvents for a wide variety of organic compounds and are also useful as reagents for producing fluorine-containing esters of carboxylic acids, in which the alcohol moiety contains fluorine substituents. A commercially important fluorine-containing alcohol is 2,2,2-trifluoroethanol which can be used to produce the known anesthetic, isoflurane, CF.sub.3 CHClOCHF.sub.2.
Methods for producing fluorine-containing alcohols, such as 2,2,2-trifluoroethanol, usually involve the reduction of esters containing this alcohol moiety. For example, U.S. Pat. Nos. 3,314,987 and 4,072,726 (to Allied Chemical Corporation) describes the hydrogenation of such fluorine-containing esters over a copper oxide-based catalyst to produce fluorine-containing alcohols and similarly, U.S. Pat. No. 3,356,746 (to Allied Chemical Corporation) describes the hydrogenation of fluorine-containing esters over supported and unsupported ruthenium and palladium catalysts.
However, processes for hydrogenating fluorine-containing alkyl carboxylic acids directly to the corresponding primary alcohols are not well known since these acids are generally more resistant to reduction than are the esters. This is particularly true for the lower members of the class such as trifluoroacetic acid and perfluoropropionic acid which are commonly used as solvents for various substrates in hydrogenation processes using platinun catalysts.
U.S. Pat. No. 3,663,629 (1972) describes a process for hydrogenating perfluoroalkane carboxylic acids at elevated temperature and pressure, in the presence of a ruthenium catalyst, thereby producing the corresponding fluorinated 1,1-dihydro alcohols. However, the process requires the presence of an initial aqueous system and is limited to the hydrogenation of C.sub.4 and higher perfluorinated alkanoic acids.
The reference, J. Org. Chem. 24, pp. 1847-1854 (1959) describes the hydrogenation of trifluoracetic acid and heptafluorobutyric acid to the corresponding primary alcohols in the presence of a rhenium black formed in situ from the heptoxide. However, the reaction requires, as a minimum, very forcing conditions, including use of a relatively large amount of catalyst calculated as rhenium (viz. 2% by weight), gaseous hydrogen pressures of about 300 atmospheres, temperatures above 200.degree. C. and a reaction time of 18.5 hours.
No mention is made in the above-described references of the use of rhodium or iridium-based catalysts for the heterogeneous hydrogenation of a carboxy group in a fluorine-containing carboxylic acid alkyl, cycloaklyl or benzene to a primary alcohol group.
Since the preparation of fluorine-containing alcohols is potentially most commercially attractive from the direct hydrogenation of the corresponding fluorine-containing carboxylic acids, what is desired is a direct one-step hydrogenation process applicable to a large class of fluorine-containing carboxylic acids proceeding at reasonable rate under mild conditions of temperature and pressure.